<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    手机号码: <input type="number"> <button>发送</button>
</body>
<script>
    var btn = document.querySelector('button');
    var time = 3;
    btn.addEventListener('click', function() {
        btn.disabled = true;
        var timer = setInterval(function() {
            if (time == 0) {
                clearInterval(timer);
                btn.disabled = false;
                btn.innerHTML = '发送';
                time = 3;
            } else {
                btn.innerHTML = '还剩下' + time + '秒';
                time--;
            }
        }, 1000);
    });
</script>

</html>